Method and apparatus for ultrasound imaging and atherectomy

ABSTRACT

A catheter for ultrasonic imaging has a transducer fixed to a cutter. The transducer is moved longitudinally within an artery while in a fixed radial position. Ultrasonic reflections are received and processed to display a planer or rectangular field of view image area of the artery. Other axial planes of the artery can be imaged by radially turning the transducer to a different angular orientation within the artery and then longitudinally moving the transducer to obtain an image of another planer field of view.

BACKGROUND OF THE INVENTION

Various apparatus and methods for intravascular ultrasound imaging andatherectomy have been known and used in the past. Yock, U.S. Pat. No.5,000,185, describes a method and catheter for performing intravasculartwo-dimensional ultrasonography and recanalization. Methods of vascularintervention by mechanical cutting have been described in Gifford etal., U.S. Pat. No. 4,669,469, Kensey, U.S. Pat. No. 4,700,705, Pope,U.S. Pat. No. 4,899,757, and Auth, U.S. Pat. No. 4,990,134. In addition,U.S. Pat. No. 4,794,931 further describes a single catheter withcombined imaging and cutting capability.

These known apparatus and methods all use a rotating imaging or cuttingelement at the distal end of a catheter. By rotating the imagingelement, for example, a transducer or a transducer/reflector assembly, areal time ultrasound image in the plane perpendicular to the vessel canbe obtained. This 360° view of the vessel enables a physician todifferentiate tissue structure and fatty deposits in the vessel wall. Bymoving the entire catheter or the imaging element in the longitudinaldirection, a three-dimensional view of the artery can be formed.

SUMMARY OF THE INVENTION

The invention relates to improved apparatus and methods forintravascular ultrasound imaging and atherectomy. An intravascularultrasound image of a plane parallel to the axis of the vessel or arteryis formed by moving an imaging element longitudinally through theartery, rather than radially. Additionally, by rotating the entirecatheter, a three-dimensional view of the artery can be formed.

Accordingly, it is an object of the invention to provide an improvedapparatus and methods for intravascular ultrasound imaging andatherectomy. Other and further objects and advantages will appearhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein similar reference characters denote similarelements throughout the several views:

FIG. 1 is a schematic illustration of a preferred embodiment of thepresent catheter and imaging system;

FIG. 2 is an enlarged side elevation view of the distal end of thepresent catheter;

FIG. 3 is a section view of the proximal end of the present catheter,including the interface to the motor drive unit;

FIG. 3A is a schematic illustration of an inductive coupler;

FIG. 3B is a cross-sectional view of the catheter connector flange whichinterfaces the catheter to the motor drive unit;

FIG. 4 is a block diagram of the image processing system;

FIG. 4A is a block diagram of an alternative pulser receiverconfiguration;

FIG. 5 is a graphic illustration of an enhancement to image processing;

FIG. 6 is a perspective view fragment of the image area provided by thepresent method and apparatus;

FIG. 7 is a schematically illustrated section view taken along line 7--7of FIG. 2;

FIG. 8 is a graphic illustration of the plane of field of view;

FIG. 9 is a diagram showing transducer movement and the correspondingimage formed on the monitor;

FIG. 10 is a perspective view fragment showing the plane of field ofview and the depth "d" of the plane;

FIG. 11 is a schematically illustrated view fragment of the distal endof the catheter, with the transducer facing the housing;

FIG. 12 is a representative example of an A-mode trace associated withthe catheter as shown in FIG. 11;

FIG. 13 is a schematically illustrated view fragment of the distal endof the catheter, with the transducer now centrally positioned in thewindow of the housing;

FIG. 14 is a representative example of an A-mode trace associated withthe catheter as shown in FIG. 13;

FIG. 15 is a representative example of an image generated upon pullingback the imaging element as shown in FIG. 11;

FIG. 16 is a representative example of an image generated upon pullingback the imaging element as shown in FIG. 13;

FIG. 17 is a representative example of an image generated upon pullingback the cutter (and transducer) with the transducer electrically activein the pulse echo mode while the cutter (and transducer) are beingrotated by the motor drive;

FIG. 18 shows a signal processing circuit for imaging during pullbackwith the motor unit turning the cutter/transducer assembly;

FIG. 19 is a flow chart illustrating a preferred method of algorithm forlinear display of catheter images;

FIG. 20 is a schematically illustrated side-view fragment of a secondpreferred embodiment of the present invention;

FIG. 21 is a schematically illustrated section view fragment of the FIG.20 embodiment;

FIG. 22 is a schematically illustrated side view fragment of a thirdpreferred embodiment of the present invention;

FIG. 23 is a similar view of a fourth preferred embodiment; and

FIG. 24 is a partial section view taken along line 24--24 of FIG. 23.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an apparatus for ultrasound imaging and atherectomyincludes a combination imaging/atherectomy catheter 30 operating over astandard interventional guide wire 32, a motor drive 34, a signalprocessing unit 36, and a video monitor 38. The catheter 30 is intendedto operate in vivo in the vascular structure and remains in the sterilefield. The guide wire 32 also remains in the sterile field and runsthrough the center of the catheter 30 and the motor drive unit 34. Fromone clinical procedure to the next, the motor drive unit is resterilizedand is kept in the sterile field during the procedure.

The motor drive 34 is connected with the signal processing unit 36 witha sufficiently long cable 40, allowing the signal processing unit toremain outside the sterile field. Typically, the signal processing unit36 may be hung from the rail on the catheter lab table, or permanentlymounted under the table. Output video signals pass from the signalprocessing unit 36 to a conveniently located video monitor 38.

FIG. 2 shows the distal end 41 of the present catheter 30, which has arigid housing 42, a flexible nose section 44 and a flexible shaft 46. Acutter 48 attached to a drive cable 50 is rotatably and axially slidablypositioned within the housing 42. The housing 42 has a window or cut-out52. A ultrasonic transducer 54 is attached to the cutter 48 to form acutter/transducer assembly 56. The transducer 54 can be a PZT crystal ora polyvinylidine fluoride material, or any composite piezoelectricmaterial. As those of ordinary skill in the art recognize, properbacking and an impedance matching layer are applied to the transducer.The exposed surface of the transducer can be flat or concave in shape. Aconcave transducer surface aids in focusing the ultrasound beam.

The catheter housing 42 is rigid and is advantageously made of metal,for example, stainless steel, to better support the cutter 48 and toprovide structural strength for the front end of the catheter 30. Thedrive cable 50 has a central lumen 51 which also passes through thecutter 48 to allow a guide wire 32 to slidably pass through the drivecable 50 and cutter 48. A balloon 57 is provided at one side of thecatheter 30 opposite the window 52. The balloon 57 can be inflated topress the window 52 of the housing 42 against a fatty deposit or plaque,such that cutter 48 can successfully shave material from the vesselwall. More than one balloon may be used, to allow proper positioning ofthe catheter.

In operation, the catheter 30 is inserted into an artery and positionedin the area of interest. The balloon(s) 57 is inflated to force thewindow 52 of the housing 42 to move toward the arterial wall and allowfatty deposits to enter the window 52. The inflated balloon(s) alsoprevents any movement of the housing 42 relative to the artery 29. It isimportant that the housing 42 be locked in position to obtain properimaging. Construction details of the catheter 30 are provided in U.S.Pat. No. 5,000,185, the disclosure of which is incorporated herein byreference.

FIG. 3 shows the proximal end of the catheter 30 and its interface withthe motor drive unit 34, in cross-sectional view. Transducer lead wires58 extend back through the catheter 30 around the drive cable 50 to thecommutator arrangement 60. The transducer lead wires 58 can also be anintegral part of or embedded in the drive cable 50. Inductive couplers,shown at 62 in FIG. 3A, may be used instead of the commutators. Theflexible shaft 46 surrounds the flexible drive cable 50 and transducerlead wires 58 which in turn surrounds the guide wire 32.

The motor drive unit 34 is intended to interface with the catheterproximal end 68 to provide rotational movement for the cutter 48,horizontal movement for the cutter 48, electrical read-out of horizontal(longitudinal) position, and electrical connection to the transducerleads 58, with minimal cost in the sterile disposable catheter portions.Specifically to that end, the proximal end 68 of the catheter comprisesa rigid connector flange 70 attached to the flexible catheter shaft 46.The housing 74 of the motor drive unit 34 is attached to the flange 70around an indented groove 72. Additionally, the drive cable 50 has asits proximal end a second rigid connector flange 76 with a retentiongroove 78 and transducer lead electrical contact plates 80 mounted onthe rear portion 82 of the connector flange 76. Rear portion 82 isformed with a non-circular shape, e.g., exhibits an octagonal,rectangular, square or other non-circular cross-section. The wires 58pass one each to the flat surfaces of the electrical contact plates 80on the outer surface of the rear portion 82 of the connector flange 76.

Within the motor drive unit 34, a rigid drive shaft 86 with an opencentral lumen for the guide wire 32 captures the drive cable connectorflange 76 with a receptacle 88. As seen in FIG. 3B, receptacle 88 isformed with a non-circular, e.g., octagonal cross-section which mateswith a rear portion 82 of the connector flange 76 to prevent relativeslippage therebetween as receptacle 88 turns. In this manner, the drivecable 50 rotates with rigid drive shaft 86. Two sets of neighboringinternal flat electrical contacts or surfaces 90 make electrical contactwith the exposed transducer lead wire electrical contact plates 80 (theshaft 86 and drive cable 50 connector flange 76 turn together).

The rigid motor drive shaft 86 is internally supported by bearings 92 attwo locations to allow rotational motion and also horizontal motion,such horizontal motion equivalent to the horizontal travel of the cutter48 within the housing 42 at the distal end 41 of the catheter 30.

Rotationally anchored to the motor drive shaft 86 is a collar 94containing a hinge 96 for a combination lever arm 98 and retention clamp100 for the drive wire connector flange 76. The retention clamp 100locks into the groove 78 to keep the drive cable connector flange 76attached to the motor drive shaft 86 while at the same time allowingsynchronous rotational motion of both shafts. Horizontal movement of thelever arm 98 causes horizontal movement of the cutter 48 within thehousing 42. The operation of the lever arm 98 is further described inU.S. Pat. No. 4,771,774, incorporated herein by reference.

Electrical lead wires 102 imbedded within the motor drive shaft 86provide a conduction path from the electrical contact points 90 in theflange receptacle 88 to rotational commutator rings 104. The transducerelectrical signals pass from the commutator rings 104 and 106 to thesignal processing unit 36 via leads 108 and the cable bundle 40.

Rotational movement of the cutter 48 in the housing 42 is provided byelectric motor 110, through first and second pinion gears 112, 114.Motor 110 is mounted to the motor drive housing 74. The first piniongear 112 is attached to the shaft of the motor 110 and drives the secondpinion gear 114 which is attached to the rigid drive shaft 86. Gear 114is longer than gear 112, allowing continuous power train drive duringhorizontal movement of the drive shaft 86. Electrical leads 118 formotor power pass to the system image processing unit via cable bundle40. An on-off switch on the motor drive unit 34 permits the user controlover rotational motion.

Attached to the lever arm collar 94 is a spring 120 for rapidly anduniformly pulling the lever arm 98 from a maximum distal position to amaximum proximal position. A friction clutch (not shown) on the collar94 controls pullback speed. A pullback latch 122 can be set and releasedto initiate automatic pullback.

The motor drive unit 34 includes a linear position encoder consisting ofa longitudinal rheostat 124. The wiper 126 or rheostat 124 is attachedto the lever arm collar 94. Lead wires 128 from the rheostat 126 andwiper 124 provide horizontal positional feedback to the system signalprocessing unit 36 via the cable bundle 40. Although shown as a separateunit in FIG. 1, the signal processing unit 36 may preferably bepositioned within the motor drive housing.

A motor drive shaft flange or thumb wheel 130 on the proximal end of themotor drive allows manual rotation of the shaft for the purpose ofaligning the transducer 54 on the cutter 48 so it is centrally locatedwithin the window 52 of the distal housing 42.

Referring to FIG. 4, within the signal processing unit 36 is anultrasound pulser 132 and a receiver 134 for respectively generating andreceiving ultrasound signals from the transducer 54 through commutator60, commutator rings 104, 106 and associated leads. The receiver 134 islinked to an A/D converter 136 which provides digital data to atwo-dimensional memory 138. A location and encoder circuit 139 connectedthrough cable bundle 40 and leads 128 to rheostat 124 reads the outputof the rheostat in the motor drive unit and converts the signal todigital bits which correspond to the longitudinal location of thetransducer, i.e., which correspond to the column location in the videodisplay of the transducer output. The digital bits indicatinglongitudinal position (i.e., indicating the video column) are suppliedto the two-dimensional memory 138. If a digital encoder, such as alinear optical encoder is used instead of rheostat 124 to determinelocation, the location encoder circuit 139 would translate the digitalencoder code to display memory location code.

Location encoder circuit 139 also supplies a "transmit trigger" pulse tothe ultrasound pulser 132. The "transmit trigger" pulse serves as asystem clock for the signal processing unit such that ultrasound pulsesare generated by the transducer at the proper instant of time. Thetrigger is also a common electrical reference point to initiate otherfunctions in the signal processing unit associated with receiving theecho stream, setting up and loading the memory, etc.

Alternately, as shown in FIG. 4A, ultrasound pulser 132 may be replacedwith a transmit encoder 140 connected to a power pulser 141 located inthe motor drive unit 134. Power pulser 141 generates the actual powerpulses for the transducer 54. Ultrasound echoes received from thetransducer are then processed in a pre-amplifier 142 in the motor drivemodule 34 before being sent to receiver 134.

Referring back to FIG. 4, the contents of the 2d memory are read by aD/A converter 143 at a rate consistent with video frame rates, passed toa video circuit 144 which translates the 2D image into an NTSC or PALvideo standard video signal, and then to conventional display monitor38. Alternatively, the 2D memory can be read and displayed on a digitalmonitor in conventional computer formats or displayed on liquid crystaldisplays by direct line-by-line read-out.

As shown in FIG. 5, the 2D memory may also consist of several memoryplanes. The displayed image can be recursively averaged as the cutter ismoved back and forth, for the purpose of smoothing the image oreliminating noise. A reset pulse from the location encoder 139 tells thememory each time the cutter direction has changed.

The 2D memory is a memory device which is partitioned into rows andcolumns. There are sufficient columns for as many columns of videopixels on the video display, and there are as many rows as there arehorizontal lines on the video display system. The memory is as deep asthere are bits per pixel on the display. For every longitudinal positionof the cutter in the housing, there corresponds a column in the memory.As the data is received, the A/D converter rate is set such thatsampling occurs for every vertical position (pixel) on the display foras many horizontal lines as exist on the display.

Referring to FIGS. 1 and 3, the speed of forward movement of the leverarm 98 is limited by clinical conditions and the user's ability to pushit forward, and is typically 5-10 seconds, to allow the cutter 48 toproperly cut tissue. The pullback throw is preferably controlled by thespring 120 and pull back latch 122 and should typically not take lessthan 0.005 seconds to complete, as acoustic data is acquired during thisphase. The complete pullback throw should be sufficient to collect allof the acoustic data, but short in time compared to anatomical motion.Alternatively, the pullback throw could be accomplished manually by theuser, or acoustic data can be acquired both on forward and backwardmotion, to continuously update the image during catheter movement.

Images are generated by sweeping the transducer back and forth. Everytime the transducer is moved a small amount, the transducer is fired andgenerates a line of information. If the transducer is moved too slowly,then anatomical motions may affect the image. If the transducer isjerked back quickly, as with a triggered retraction, then all of theacoustic information can be obtained in a time frame small compared tothat associated with body movements.

FIG. 6 shows the longitudinal area 146 imaged with the present methods.In one method, the cutter 48 carrying the transducer 54 is rotated,while in another method the transducer is not rotated and is maintainedin alignment with the window. During imaging, it is important to avoidnon-linear movement of the cutter/transducer assembly relative to theartery 29. By inflating the balloon(s) and quickly pulling back thecutter/transducer assembly, especially during the diastolic movement ofthe heart function, the desired linear movement can be readily achieved.

As shown in FIGS. 7-17, the combined imaging/atherectomy catheter of thepresent invention is used to acquire images in a longitudinal plane. Ascan best be seen in FIGS. 7-10, the longitudinal plane extends from thetransducer to a chosen depth "d", and along the catheter z-axis for theallowable movement range of the cutter/transducer assembly. In practice,this may be from less than one centimeter to a maximum of about 2centimeters. Display vectors are parallel to one another and theirspacings are determined by circuitry which determines the steps in thez-axis movement. When used with a rotating transducer, an index pulseserves to position the vector in the plane exactly within the window 52.The result of this timing is to define a plane positioned at apredetermined angle Θ of the rotating transducer, which is formed by thevectors of depth "d" moving along the z-axis of the catheter/transducer.

An image of the tissue or other anatomical structure of interest whichintersect the plane described above is made up of digitized ultrasounddata taken from each vector. This data is displayed either with vectorsoriented vertically, or with vectors oriented horizontally along theraster of the output device.

As shown in FIG. 11, the transducer 54 on the cutter 48 within thehousing 42 of the combined imaging/atherectomy catheter is directedtoward the wall of the housing. The imaging ultrasound beam 148 cannotpropagate acceptably through the metal of the housing and in fact mostof the energy will reverberate between the transducer and the housingwall. FIG. 12 depicts a typical A-mode tracing of the received acousticwaveform, showing the ringdown 150 of the excitation, the first acousticecho from the housing wall 152, the first reverberation off the housingwall 154, and subsequent reverberations 156, 158, 160 off the housingwall.

If the cutter 48 is rotated by the drive cable 50 and thumbwheel (or themotor) such that the acoustic beam 148 is now centrally positioned inthe window of the housing, as shown in FIG. 13, the acoustic beam 148 isthen free to propagate through the human tissue and might present withan A-mode trace as shown in FIG. 14, where 162 represents the sameexcitation ringdown as seen in FIG. 12 (ringdown 150), with low signals164 due to scatter from blood and enhanced echoes 166 and 168 dueperhaps from tissue structures such as the internal and external elasticlamina of the vessel wall.

With the cutter in the position shown in FIG. 11, such that the acousticbeam 146 strikes and reverberates off the housing wall, pullback of thecutter for the purpose of generating an image will result in an imageillustrated in FIG. 15, where the horizontal line 170 corresponds to theexcitation ringdown 150, the horizontal line 172 to the first echo 152off the housing wall, horizontal line 174 to the first reverberation 154off the housing wall, etc. The vertical line 182 corresponds to thecurrent position of the transducer in its pullback through the length ofthe window, with the dashed lines to the right indicating anticipatedimages due to the continued reverberation.

With the cutter rotated to align the transducer within the window suchthat the acoustic beam freely passes through the window, one might seean image of the type shown in FIG. 16, where the horizontal line 190represents the excitation ringdown 162 as in FIG. 14, and the remainingfeatures are identified as the absence of echo 192 corresponding to theblood region with minimal echo 164 as seen in FIG. 14, the internalelastic lamina line 194 and the external elastic lamina line 196corresponding to echos 166 and 168 of FIG. 14. The vertical line 198represents the current position of the transducer, with the image to theleft representing past history (anatomy just scanned) and the dashedlines to the right representing anticipated images from anatomy to bescanned.

In the preferred embodiment of this invention, with the intent ofkeeping the hardware and operation simple, the user would adjust theangular orientation of the cutter such that the transducer werepositioned midway in the window, by the manual rotation of the thumbwheel at the motor drive assembly. The thumb wheel communicates with thecutter via the drive cable 50, with the drive cable having sufficienttorsional rigidity so as to provide one-to-one rotation between thethumb wheel and the cutter. The user would use the acoustic signature onthe screen as a means to determine when the cutter has achieved theproper orientation.

Should the transducer be electrically active in the pulse echo modewhile the cutter is being rotated by the motor drive and while apullback is taking place, one might expect an image as illustrated inFIG. 17. Horizontal trace 200 represents the excitation ringdown of thetransducer; horizontal lines 202, 208, 202, 208 represent acousticimages of the housing wall and the first, second and thirdreverberations, respectively; and generally diagonal lines 210 and 210represent the internal and external elastic lamina of the vessel,respectively. Vertical lines 214 represent the transition as theacoustic beam passes from a reverberation mode bouncing off the housingwall to free imaging through the window of the housing. Vertical lines216 represent the subsequent transition, as the cutter continues torotate and the transducer again faces the housing wall, with theacoustic beam reverberating off the wall.

For imaging the interior vessel wall with the motor turning the cutterassembly, and with simultaneous pullback, the signal processing circuitshown in FIG. 4 is modified, as in FIG. 18. The oscillator 218 in FIG.18 causes the pulser to fire at a frequent, regular interval. A typicalpulse rate is on the order of 30 KHz. The echo pattern is passed back tothe signal processor receiver, where the signal is detected andconverted to digital form by the A/D converter. The digital signalpasses to the 2D memory in the same manner as in FIG. 4.

The detected signal also branches (either before the A/D converter inthe analog form, or after the A/D converter in the digital form) to awaveform recognition circuit 220. In this circuit, the waveform passesto a correlator circuit, and is compared to the previous waveform (whichis passed to the correlator circuit at the same time, after having beendelayed by one vector or pulse rate period). If the two waveforms match,the transducer has seen the wall of the housing twice in a row, andtherefore is still at the incorrect orientation for vessel wall imaging.The correlator enables the "Delay N Vector" circuit, in turn setting the"Inhibit Gate" circuit to send an INHIBIT signal, which prevents thelocation encoder from updating the 2D memory, and consequently preventsreceived echo data from entering the memory for display. If thecorrelator does not see a match between two consecutive waveforms, theacoustic beam is either entering the window of the housing, is in thewindow and seeing different anatomical structures, or is exiting thewindow. Upon this condition, the INHIBIT signal is deactivated for onetransducer pulse period, at a specified time delay after first sensingthis condition, with such delay provided to allow the acoustic beam toreach the center of the window. Operation of the waveform recognitioncircuit 220 is timed from the "transmit trigger" pulse output from thelocation encoder circuit 139.

Without the INHIBIT signal, the encoder location position is passed tothe 2D memory and the received data vector in the digital format iswritten at the appropriate location in the memory. If pullback is slowerthan appropriate with respect to the rotation of the cutter, individualdata vectors in the 2D memory will be overwritten, without degradationto image quality. If the pullback is too fast, the image will presentwith gaps.

The 2D memory might be configured as a dual port RAM. At time framesconsistent with scanning, the memory would be loaded, irrespective ofdisplay frame rates. At time frames consistent with the video formats,the memory would be read, converted to analog video format, andtransmitted to the appropriate video display hardware, in the givenvideo standard.

Another embodiment causes the motor driver to stop rotation as a resultof the INHIBIT/NON-INHIBIT transition so as to cause the cutter to be sooriented that the acoustic beam passes through the center of the window,allowing tissue imaging during the entire length of the pullback. Inthis mode, signal processing would occur as if the cutter had beenaligned with the window by the operator.

FIG. 19 depicts a flow chart outlining the operation of signalprocessing unit 36. Each vector is generated by a trigger module (notshown) in the location encoder circuit 139 which takes its input fromthe z-axis movement of the cutter/transducer. The trigger module isresponsible for introducing the appropriate time delays associated withthe transducer geometry. A positioning mechanism is necessary to keeptrack of each new vector being updated.

The algorithm used to acquire and display the ultrasound image data isas follows:

1. Initialize at power on.

2. Enter main loop which scans user input and implements the selection.Set gain, TGC, display presentation adjustments.

3. Continue in main loop.

4. Wait for user catheter selection;

a. set defaults based on frequency, depth, linear scan distance, burst.

b. paint markers etc. on display,

5. Wait for user command;

a. input from panel.

b. input from positioning mechanism. Set current vector position ondisplay (which could be mid-screen).

6. Start command

a. start rotation (if rotating transducer is used)

b. input from positioning mechanism.

(i) synchronize timing using index pulse

(ii) activate trigger module

(iii) fire burst

(iv) delay onset of acquisition

c. begin acquisition of ultrasound data (detected/filtered.)

(i) use sample frequency set by defaults

(ii) convert new sample and store in display buffer (in the case ofvertically displayed vectors)

(ii-a) use vector number as column address

(ii-b) use sample number as row address

(ii-c) continue until sample depth is reached

d. input from panel (look for stop command)

7. Stop command

a. stop rotation (if rotating transducer is used)

An alternate embodiment of the present catheter is shown in FIGS. 20 and21. In this embodiment, a separate transducer assembly 250 is positionedbehind the cutter 48. The transducer assembly 250 includes a transducer54 mounted on a transducer support 252. A dovetail 254 on the support252 locks into the window of the housing. Accordingly, the transducerassembly 250 cannot rotate. Hence, the electrical connections to thetransducer are stationary as well and no commutator or inductivecoupling is required. A spacer 256 is attached to the drive cable 50between the back of the cutter and the transducer assembly 250, and acollar 258 is attached to the drive cable 50 on the opposite side of thetransducer assembly. The spacer 256 and collar 258 keep the transducerassembly from sliding or shifting longitudinally with respect to thecutter. The transducer assembly can be moved by the windowlongitudinally and the transducer remains properly oriented or radiallyaligned with the window. The ringdown effect, if any, can be resolved inthe embodiments of FIGS. 20 and 21 by positioning the transducer in arecess on the cutter or the transducer support.

In another embodiment, transducers 54 are placed on the housing 42 asshown in FIG. 22. In this embodiment, the entire catheter must belongitudinally moved to obtain a two-dimensional image. Cross-sectionalimages can be obtained by rotating the entire catheter. Since theballoon 64 cannot be inflated while moving the catheter, no locking isprovided. The embodiment of FIG. 22 is therefore more useful in slowermoving arteries, such as those in the legs. Arteries in the legs aretypically larger in diameter and straighter than other arteries in thebody; a more rigid catheter can be used and more easily controlled inleg arteries. Accordingly, with an imaging transducer on the housingoutside surface, an image can be created by moving the catheter, insteadof moving a carriage or transducer carrier within the catheter.

In yet another embodiment, as shown in FIGS. 23 and 24 a thin filmtransducer 260 is placed on the inside wall of the housing 42 oppositeto the window 52. Imaging is performed when the cutter 48 is moved outof the view of the transducer 260. The field of view of the transducer260 in this embodiment is relatively narrow. Longitudinal and rotationalmovement of the catheter can be performed to obtain wider views of theartery.

In the embodiment of FIG. 1, the transducer 42 moves longitudinally withthe cutter 48. This movement of the transducer is measured by theencoder from the displacement of the drive cable 50 with respect to theouter wall of the catheter 30. On the other hand, in the embodiments ofFIGS. 22-24, since the transducer is attached directly to the catheterhousing, movement of the catheter itself must be measured with respectto the patient's body. This is achieved by a catheter movement measuringdevice (not shown) linked or attached to the catheter and theelectronics unit.

Although several embodiments have been shown and described, it will beapparent to those skilled in the art that many modifications andvariations can be made thereto without departing from the spirit andscope of the present invention.

What is claimed is:
 1. In a method for ultrasonic imaging of the typeincluding the step of rotating a transducer within a housing having awindow, the improvement comprising the steps of:detecting when thetransducer is at the center of the window; and displaying an imagederived from reflections received by the transducer, when the transduceris at the center of the window.
 2. The method of claim 1 furthercomprising the steps of:measuring an angle between a first edge of thewindow and the transducer; and comparing the measured angle to apredetermined angle equal to one-half of the angle subtended by thewindow, to detect when the transducer is at the center window.
 3. Acatheter for ultrasonic imaging and atherectomy within a vesselcomprising:a flexible catheter body; a flexible drive cable disposedwithin the catheter body; a transducer housing mounted at the distal endof the catheter body, said transducer housing having a window; a cuttingassembly attached to the flexible drive cable and disposed within thetransducer housing so that it can cut material protruding into thetransducer housing from without; a transducer mounted on the cuttingassembly, said transducer being operably connectable to an ultrasonictransmitter, receiver, signal processing unit, and display unit so thatsaid transducer can ultrasonically scan the vessel to create images ofthe vessel; means for longitudinally advancing and retracting thecutting assembly; means for detecting longitudinal transducer positionof the transducer and communicating said transducer position to thesignal processing unit for use in constructing an image of the vessel; arigid connector flange mounted on the proximal end of the drive cable; arigid drive shaft having a receptacle at its distal end, said receptaclebeing sized and dimensioned so as to receive the rigid connector flangein such a manner as to ensure that the drive cable and rigid drive shaftwill rotate together; a rotational drive motor operably connected to therigid drive shaft and capable of rotating the rigid drive shaft therebyrotating the drive cable; and releasable retention means forlongitudinally retaining the rigid connector flange within thereceptacle.
 4. The catheter of claim 3 wherein the means forlongitudinally retracting the cutting assembly comprises a springattached to a lever arm.